1. Filed of the Invention
The present invention relates to a solid-state imaging device having a high quality image and low power consumption, and a method of driving the same.
2. Background
As solid-state imaging devices mounted in cellular phones and so forth, there are a CCD (charge coupled device) type image sensor and a CMOS type image sensor. The CCD type image sensor is excellent in image quality, while the CMOS type image sensor consumes lower power and its process cost is low. In recent years, MOS type solid-state imaging devices using a threshold voltage modulation method that combines both high quality image and low power consumption have been proposed. The MOS type solid-state imaging device of the threshold voltage modulation method is disclosed in Japanese Unexamined Patent Publication No. 2002-134729, for example.
In the solid-state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, image output is obtained by arranging unit pixels in a matrix and repeating three states of initialization, accumulation, and reading. Moreover, in the solid-state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, each unit pixel has a photo-diode, a modulation transistor, and an overflow drain gate. The gate of the modulation transistor is formed in a ring shape.
Charges (photo-generated charges) generated by light incident upon a photo-diode are transferred to a P-type well region provided under a ring gate so as to be accumulated in a carrier pocket formed in this region. The threshold voltage of the modulation transistor changes corresponding to the photo-generated charges accumulated in the carrier pocket. Thus, a signal (pixel signal) corresponding to incident light is obtained from a terminal coupled to the source region of the modulation transistor.
In the solid-state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, in order to prevent distortion caused when imaging a moving object, an image by an optical signal is captured at the entire light-receiving surface, and then the optical signal is converted into an electric signal so as to be taken out to the outside as an image signal. However, in this reading method, an accumulating period and a reading period cannot be controlled for each pixel because an image by an optical signal is captured at the entire light-receiving surface. Therefore, while specific pixels are being read, the accumulating operation cannot be carried out in other pixels. As a result, the frame rate cannot be enhanced.
In addition, in the solid-state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, photo-generated charges accumulated in a carrier pocket are discharged toward an area under a P-type well via an N-type layer at an initialization time. In other words, the photo-generated charges are discharged toward a substrate via an N-type layer. Therefore, the P-type well having a carrier pocket is required to hold carriers in a reading period, and to discharge the carriers in a discharging period. In order to satisfy such a trade-off need, the thickness and impurity concentration of each impurity layer needs to be strictly controlled, and hence the versatility in the design is remarkably impaired.
Furthermore, in the solid-state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, part of the photo-generated charges generated in a photo diode are discharged toward a substrate via an overflow drain region formed of a P-type layer.
Incidentally, in an N-type layer constituting a photo diode, as the peak of impurity concentration becomes deeper perpendicularly downward in the substrate, the wavelength range of photoelectric-convertible incident light becomes wider. That is, taking a higher quality image into consideration, the depth of the N-type layer needs to be deep enough. Meanwhile, the overflow drain region is made up of a deep P-type layer extending from under an overflow drain gate to the back surface of the substrate. This P-type layer is formed by implanting P-type impurities after forming the above N-type layer. Therefore, in order to form the overflow drain region made up of a deep P-type layer, ion-implanting energy needs to be large. Generally, the area of an impurity region formed with increased ion-implanting energy becomes large. That is, the area of the overflow drain region becomes large. Therefore, there is a trade-off relationship between a higher quality image and miniaturization, and there is also a problem in that a solid-state imaging device cannot be miniaturized if the N layer is deepened for achieving a higher quality image.
The present invention is made in view of such problems, and is intended to provide a solid-state imaging device and a method of driving the same that can attain a higher quality image and miniaturization without impairing the versatility in the design while enhancing the frame rate by allowing an accumulating period and a reading period to be set to a common timing.